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CXCR4
CXCR4 is also suggested to be involved in adult neurogenesis.64–66 There is a loss of mitotic, nestin+(effective)
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A type VI intermediate filament (IF) protein, expressed mostly in nerve cells where it is implicated in the radial growth of the axon
, and (DCX)+ cells in the dentate gyrus when a CXCR4 antagonist, AMD3100, is infused.
From: Progress in Molecular Biology and Translational Science, 2013
Related terms:
B Cell
T Cells
Chemokine
CCR5
CD4
Chemokine Receptors
Nested Gene
Hematopoietic Stem Cells
Human Immunodeficiency Virus
Human Immunodeficiency Virus 1
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CHEMOKINES, CXC | CXCL12 (SDF-1)
R.M. Strieter, B.N. Gomperts, in Encyclopedia of Respiratory Medicine, 2006
Receptor
CXCR4 is a G-protein-coupled seven transmembrane receptor that was originally cloned as an orphan chemokine receptor and was known as LESTR or fusin. CXCR4 is expressed on the cell surface of most leukocytes, including all B cells, and monocytes and most T lymphocyte subsets, but just weakly on NK cells. It is also expressed on nonhematopoietic cells such as endothelial cells and epithelial cells, and adult stem cells such as fibrocytes and circulating progenitor epithelial cells. CXCR4 is also an essential cofactor for T-tropic HIV-1 and HIV-2 env-mediated fusion and entry into CD4+ lymphocytes. Recently, a second alternatively spliced CXCR4 receptor was cloned and named CXCR4-Lo. CXCR4-Lo has lower gene expression in most tissues than CXCR4 except in the spleen and lung; its function is not yet known.
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Immunotherapy of Cancer
Qianyu Guo, ... Wilson H. MillerJr, in Advances in Cancer Research, 2019
5.2.3 CXCR4 inhibitor
CXCR4 is the cell surface receptor for CXCL12 (stromal cell-derived factor-1 α, SDF1α). CXCL12/CXCR4 interactions transduce signals downstream to the PI-3K/Akt, MAPK and JAK/STAT pathways (Chatterjee, Behnam Azad, & Nimmagadda, 2014). CXCR4 has been implicated in cell transformation and early stage of tumorigenesis (Shiah et al., 2015). CXCR4 overexpression, which has been identified in multiple cancer types, also supports cancer metastasis, recurrence and therapeutic resistance (reviewed by Chatterjee et al., 2014). More importantly, CXCR4 has been shown to enhance tumor immune evasion by recruiting Treg (Gil et al., 2014) and MDSCs (Obermajer, Muthuswamy, Odunsi, Edwards, & Kalinski, 2011), to promote angiogenesis (Orimo et al., 2005), and to support CAF-dependent immunosuppression (Orimo et al., 2005).
Pre-clinical studies have provided evidence to support the anti-tumor effect of CXCR4 inhibition in conjunction with various other therapies. For example, the CXCR4 inhibitor AMD3100 can sensitize prostate cancer cells to docetaxel (Domanska et al., 2012), and the combination of AMD3100 and sorafenib with PD-1 blockade results in T cell-based elimination of hepatocellular carcinoma (Chen et al., 2015). Ongoing clinical trials are testing CXCR4 inhibitor-based therapy in combination with other anti-cancer agents (Table 3).
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Emerging Applications of Molecular Imaging to Oncology
Samit Chatterjee, ... Sridhar Nimmagadda, in Advances in Cancer Research, 2014
4.7 Gastrointestinal cancers
CXCR4 is involved in tumor growth and metastasis in various gastrointestinal cancers, in particular colorectal, pancreatic, hepatocellular, gastric, and esophageal cancers.
Each year, more than one million people are diagnosed worldwide with colorectal cancer (CRC), which is the fourth most common cause of cancer-related death (WHO, 2012). In colorectal cancer patients, CXCR4 expression in primary tumor cells correlates with survival, metastasis, and recurrence (Kim et al., 2006). All the CRC samples stained for CXCR4 by IHC were positive with nearly 58% demonstrating strong expression (Schimanski et al., 2005). Similarly, analysis of cell lines, 100 CRC tumors and 39 liver metastases by qRT-PCR demonstrated higher CXCR4 expression in the cell lines and tumors (Kim et al., 2005). Patients with high CXCR4-expressing tumors had increased risk of local recurrence and distant metastases, lymph node involvement, as well as significantly decreased OS (median, 9 months vs 23 months; log-rank P = 0.03) (Kim et al., 2005). Also, CXCR4 expression was higher in the liver metastases compared to primary tumors. In 12 of 14 paired tumors and metastases, CXCR4 expression was higher in the metastases than the primary tumor (Kim et al., 2005; Schimanski et al., 2005). These observations reiterate the role of CXCR4 expression in CRC growth, recurrence, and metastasis. Studies in an animal model using CT-26, a mouse colorectal cancer cell line, revealed that CXCR4 is important for metastasis of colon cancer to liver but not involved in tissue invasion (Zeelenberg, Ruuls-Van Stalle, & Roos, 2003). Interestingly, CXCR4 surface expression levels were found to be low or absent in colon cancer cell lines in vitro while high expression levels were observed in vivo in animal models of liver metastasis (Zeelenberg et al., 2003). These findings suggested that CXCR4 expression on colon cancer cells is regulated by tumor microenvironment and isolated metastatic cells utilize CXCR4 signaling for proliferation.
Pancreatic cancer has a very poor prognosis and limited early detection options with a 5-year survival of less than 5% (ACS, 2014). In pancreatic cancer, the CXCR4/CXCL12 axis plays an important role in tumor cell proliferation, migration, and angiogenesis. Nearly 85% of pancreatic tumor samples tested were positive for CXCR4 expression. Patients with high CXCR4-expressing tumors had a worse outcome than those with low CXCR4 expression with OS: 9.7 months (95% CI: 6.0–13.4) versus 43.2 months (95% CI: 16.3–78.1), P = 0.0006 (Marechal et al., 2009). In another study, high CXCR4 expression in pancreatic adenocarcinoma was observed to be an independent negative prognostic biomarker (HR = 1.74; P < 0.0001) and associated with distant relapse (HR = 2.19; P < 0.0001) (Bachet et al., 2012, 2012). Also, a subpopulation of CSCs expressing CD133 and CXCR4 in invasive pancreatic tumors was found to be the determinant of metastasis (Hermann et al., 2007). While HIF-1α is known to be a major factor contributing to CXCR4 expression in pancreatic and other cancers, recent studies in pancreatic cancer cells and tumors demonstrated that transcription factors such as SOX9 upregulate CXCR4 expression independently of HIF-1α, which may have consequences not only for pancreatic cancer but also for other cancers such as SCLC where SOX transcription factors are known to be overexpressed (Camaj et al., 2014). CXCR4 antagonist AMD3100 significantly inhibited the proliferation, migration, and invasion of pancreatic cancer cells (Gao, Wang, Wu, Zhao, & Hu, 2010). Mori et al. (2004) reported that CXCR4 antagonist TN14003 inhibited the migration and invasion of pancreatic cancer cells. Singh, Srivastava, Bhardwaj, Owen, and Singh (2010) demonstrated that inhibition of the CXCR4/CXCL12 axis by AMD3100 arrested the pancreatic cancer cell growth and abrogated gemcitabine resistance. Ma, Hwang, Logsdon, and Ullrich, 2013 showed that AMD3100 treatment reduced tumor growth in animal models of pancreatic ductal adenocarcinoma by blocking CXCR4-dependent mast cell migration.
Hepatocellular carcinoma (HCC) is one of the most common cancers and causes 745,000 deaths each year (WHO, 2014b). Roughly, 50% of HCC tumor specimens were identified as CXCR4 positive (Xiang et al., 2009). In HCC, the expression of CXCR4 was found to be correlated with tumor progression, lymphatic metastasis, distant dissemination, and a reduced 3-year survival rate (Schimanski et al., 2006). The CXCR4/CXCL12 axis was reported to regulate angiogenesis, essential for growth and progression of HCC (Li, Gomez, & Zhang, 2007). Li et al. (2007) found a higher expression of the CXCL12 and CXCR4 in sinusoidal endothelial cells in HCC specimens than in normal liver tissues. Findings by Mavier et al. (2004) suggested that CXCR4/CXCL12 axis promotes the proliferation of oval cells and abnormal differentiation of these cells may be associated with HCC. A study by Chu et al. (2007) indicated that the CXCR4/CXCL12 axis mediates active MMP-9 and MMP-2 secretion, thereby facilitating metastasis. CXCR4 inhibition by AMD3100 in combination with sorafenib treatment was reported to inhibit HCC growth (Chen et al., 2014).
In esophageal cancer, CXCR4 expression was found to be correlated with increased lymph node and bone marrow metastases (Sasaki et al., 2008). Approximately 85% of esophageal cancer tumors are CXCR4 positive (Sasaki et al., 2009). A study by Gockel et al. (2006) showed that patients with CXCR4-expressing tumors have a lower median OS of 20 months compared to a median OS of 76 months for patients with CXCR4-negative tumors. Supporting these observations, CXCR4 gene silencing by lentivirus shRNA inhibited proliferation of the EC9706 human esophageal carcinoma cell line and reduced the growth of tumor xenografts in mouse models (Wang, Lou, Qiu, Lin, & Liang, 2013).
Gastric and stomach cancers cause 723,000 deaths every year (WHO, 2014b) and have a poor prognosis with less than 10% 5-year survival rate (Orditura et al., 2014). Positive staining for CXCR4 was identified in 80% of the primary gastric tumor tissues (Han et al., 2014). CXCR4 expression in primary gastric carcinomas is associated with the development of peritoneal carcinomatosis and malignant ascites which contained high levels of CXCL12 (Yasumoto et al., 2006). CXCR4 expression in primary gastric tumors was positively correlated with lymph node metastasis (Ying, Xu, Zhang, Liu, & Zhu, 2012). A meta-analysis by Han et al showed that CXCR4 expression is associated with poor prognosis in gastric cancer patients. In this study, OS was found to significantly correlate with CXCR4 expression, with the HR of 2.63 (95% CI: 1.69–4.09; P < 0.0001), and a significant association was also detected between CXCR4 expression and tumor stage (odd ratio (OR): 0.52, 95% CI: 0.32–0.83; P = 0.007), depth of invasion (OR: 0.44, 95% CI: 0.27–0.73; P = 0.001), lymph node metastasis (OR: 2.30, 95% CI: 1.57–3.36; P < 0.0001), and vascular invasion (OR: 0.72, 95% CI: 0.53–0.98; P = 0.04) (Han et al., 2014). Fakhari et al. (2014) reported that Helicobacter pylori infection increased CXCL12 secretion by gastric epithelial cell line, upregulated CXCR4 expression in bone marrow-derived-mesenchymal stem cells, and enhanced their migration toward CXCL12 gradient. Findings by Oh et al. (2012) indicate that hypoxia upregulates CXCR4 in gastric cancer cells in a HIF-1α-dependent manner and that upregulation of CXCR4 is involved in gastric cancer cell migration and invasion. Iwanaga et al. showed that CXCR4 blockers AMD3100 and KRH3955 inhibited the growth of gastric cancer xenografts in a mouse model (Iwanaga, Iwasaki, Ohashi, Nunobe, & Iwagami, 2007; Iwanaga et al., 2012).
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HIV/AIDS
S. Kaushik, J.A. Levy, in Encyclopedia of Microbiology (Third Edition), 2009
CXCR4
CXCR4 acts as a coreceptor for T cell-line-tropic HIV strains, permitting a closer interaction between the virus and the cell surface. The amino terminal domain of CXCR4 is involved in HIV binding, especially the second extracellular loop structure. The viral V3 loop is involved in X4 virus infection. The natural ligand for CXCR4 is the chemoattractant stromal-derived factor 1 (SDF1), which can block HIV infection of T cells. The X4 strains induce multinucleated syncytia in T cell lines like MT-2, and hence are called 'syncytia-inducing (SI) viruses'.
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Interleukin-8 and other CXC chemokines
Naofumi Mukaida', ... Kouji Matsushinur, in The Cytokine Handbook (Fourth Edition), 2003
CXCR4
Human CXCR4 consists of 352 amino acids (Federsppiel et al., 1993; Nomura et al., 1993; Loetscher et al., 1994) and its gene is localized on human chromosome 2q21 (Herzog et al., 1993). CXCR4 binds CXCL12 exclusively with a high affinity and is expressed on hematopoietic progenitor cells, B cells, T cells, endothelial cells and dendritic cells (Bleul et al., 1996b; Oberlin et al., 1996). Ablation of CXCR4 gene results in embryonal or perinatal death with defective development of cardiac ventral septae, large vessels and cerebellar architecture (Tachibana et al., 1998; Zou et al., 1998), indicating the indispensable roles of the CXCR4-CXCL12 axis in organogenesis. However, the functions of CXCR4 in adults remain to be investigated.
Some human immunodeficiency virus (HIV)-1 strains utilize CXCR4 along with the CD4 antigen (Berger et al., 1998). These strains are now called X4 strains and are typically isolated late in the course of the infection and correlate more or less with T-cell line tropism. gp120 from the HIV envelope glycoprotein can bind to CXCR4 in the presence of CD4 (Lapham et al., 1999), although CD4-independent association of gp120 with CXCR4 has been demonstrated. While purified X4 gp120 inhibits monocyte response to CXCL12 through the interaction with CXCR4 (Wang et al., 1998), it can induce the apoptosis of the human neuronal cell line through the interaction with CXCR4 on the cell surface (Hesselgesser et al., 1998; Zheng et al., 1999). These findings may be relevant to the pathogenesis of HIV encephalitis and AIDS dementia.
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Ubiquitination and Transmembrane Signaling
P.-Y. Jean-Charles, ... S.K. Shenoy, in Progress in Molecular Biology and Translational Science, 2016
2.2.2 CXCR4
The GPCR CXCR4 is activated by the endogenous chemokine CXCL12 (also called stromal cell-derived factor 1 alpha, SDF1α) and regulates migration or chemotaxis of different cell types including leukocytes and cancer cells.136,146,147 CXCR4 acts as a coreceptor for HIV-1 entry into T cells and CXCL12 has been shown to inhibit HIV-1 entry into T cells. CXCR4 primarily couples to the Gi family of heterotrimeric G proteins and is phosphorylated by both PKC and GRKs. However, CXCL12 stimulation primarily leads to GRK-mediated phosphorylation on specific residues in the C-tail of CXCR4.148 Although both nonvisual arrestins associate with activated CXCR4, β-arrestin2 is recruited more readily than β-arrestin1. β-arrestin-binding requires GRK2-mediated phosphorylation of a distinct group of serines in the C-tail of CXCR4.148 β-arrestin-dependent ERK activation proceeds only when β-arrestin binds CXCR4 that is phosphorylated on specific serine residues by GRK3 and GRK6.148 CXCR4 downregulation requires lysosomal trafficking and a tight regulation of CXCR4 expression is required for normal physiology. An increase in CXCR4 expression (perhaps caused by a faulty degradation pathway) is seen in many aggressive cancers, which require CXCR4 signaling for growth and metastasis.
CXCL12 stimulation induces monoubiquitination of CXCR4, which is dependent upon the phosphorylation of serines within a degradation motif "SSLKILSKGK" in the C-tail of the receptor.55 Mutation of either serines or lysines in this motif affects both ubiquitination and ligand-induced degradation of CXCR4. The HECT-domain E3 ligase called Atrophin-1-interacting protein 4 (AIP4) ubiquitinates CXCR4 and binds the receptor C-terminus via a direct interaction that involves seryl-phosphorylation of CXCR4 and WW domains in AIP4.149 CXCR4 lysosomal degradation also requires AIP4-mediated ubiquitination of the ESCRT-0 protein, HRS.54 β-arrestins are not required for AIP4-mediated ubiquitination of CXCR4. However, β-arrestin1 is required for the lysosomal trafficking of CXCR4.53 The N-terminus of β-arrestin1 interacts with the WW domains of AIP4. Furthermore, β-arrestin1 interaction with the endosomal protein STAM-1 (signal transducing adaptor molecule 1) is required for HRS ubiquitination, but not for CXCR4 or STAM-1 ubiquitination.56 Although β-arrestin2 binds CXCR4 and HRS, it does not interact with STAM-1, hence the ESCRT-dependent sorting of CXCR4 appears to be predominantly affected by β-arrestin1.56,116 AIP4 and STAM-1 also associate directly through a polyproline region in AIP4 and an SH3 domain in STAM-1.150 Ubiquitination of STAM-1 by AIP4 in caveolae is required for CXCR4-induced ERK activation.150 The activity of AIP4, which is critical for the ubiquitination of CXCR4 and ESCRT proteins is suppressed when the expression of a RING domain E3 ubiquitin ligase Deltex3L is elevated.151 In addition to Deltex3L and β-arrestin1, whether AIP4 and its activity in CXCR4 endocytic trafficking involves the ARRDC proteins remains to be defined.
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HSC Niche
Samiksha Wasnik, ... David J. Baylink, in Biology and Engineering of Stem Cell Niches, 2017
3.1.3 CXCR4 Antagonists
The CXCR4/SDF-1 axis is essential for HSC retention within the BM niche. SDF-1 is a ligand for CXCR4 chemokine receptor (CXCR4), which is mainly expressed by immature osteoblasts, endothelial cells,35 and CXCL12-abundant reticular (CAR) cells.36,37 HSCs and HPCs express CXCR4 and are chemoattracted to the high level of SDF-1. Disruption of the CXCR4/SDF-1 axis, such as conditional deletion of CXCR4 in mice, resulted in reduced number of HSCs and increased sensitivity to myelotoxic injuries,37 as well as significantly increased HPCs in the peripheral circulation.38 The elevation of SDF-1 levels in the peripheral blood by intravenous injection of an adenoviral vector expressing SDF-1 resulted in the mobilization of HPCs and HSCs from BM to the peripheral circulation.39
AMD3100 is the only FDA-approved CXCR4 antagonist40 for stem cell mobilization. In combination with G-CSF, AMD3100 mobilizes HPCs with high clinical efficacies and few side-effects.39 Many other small molecules/peptides have been developed to target the CXCR4/SDF-1 axis for mobilization of HPCs. POL6326 is a selective and reversible CXCR4 inhibitor. A single injection of POL6326 leads to an 11- to 12-fold increase in circulating progenitor cells.41 BKT-140 is a 14-residue peptide and functions as an inverse agonist and binds CXCR4 residues in the extracellular domains.42 NOX-A12 is an L-enantiomeric RNA oligonucleotide that binds and antagonizes CXCL12.43 Preclinical studies reported the dose-dependent mobilization ability of NOX-A12. Many other CXCR4/SDF-1 antagonists areunder development and in preclinical/clinical evaluation, including MDX-1338,43 FC131,44 WZ811, POL5551,45 etc.
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Neuroimmune Signaling in Drug Actions and Addictions
Bradley Nash, Olimpia Meucci, in International Review of Neurobiology, 2014
3.1 Physiological and pathological roles of CXCR4
CXCR4 is expressed in the brain and the spinal cord in vitro and in vivo in a vast variety of species (Meucci et al., 1998; Ohtani et al., 1998; Pitcher et al., 2014) and in all major CNS cell types, including neurons (Meucci et al., 1998; van der Meer, Ulrich, Gonzalez-Scarano, & Lavi, 2000), astroglia (Bajetto et al., 1999), microglia (Lipfert, Odemis, Wagner, Boltze, & Engele, 2013), and oligodendrocytes (Maysami et al., 2006). The receptor has the potential to activate several distinct signaling pathways and elicit various biological responses. The natural ligand CXCL12 binding results in inhibition of adenylate cyclase, via activation of Gαi proteins, and decreased protein kinase A activity (Zheng et al., 1999)—while also increasing intracellular Ca2 + levels and protein kinase C, via the phospholipase C pathway (Cali & Bezzi, 2010; Khan et al., 2004; Meucci et al., 1998). Additional downstream signals with direct effect on gene transcription include activation of the ERK and Akt cascade (Khan et al., 2004), the JAK/STAT, and the nuclear factor-κB pathways (Ganju et al., 1998). As a homeostatic chemokine/receptor pair, these proteins have much more varied roles than contributing to immune responses compared to their inflammatory counterparts. The signaling outcomes of the receptor are indeed similar to the classical immune chemotactic response, but the same signals can also occur on nonimmune cells that express CXCR4. CXCR4 and CXCL12 are expressed in both the developing and mature CNS, where they serve multiple vital functions. For instance, during development, CXCL12 guides developing interneurons to their proper cortical layer via a CXCL12 gradient that is produced by resident cells (Sanchez-Alcaniz et al., 2011; Stumm et al., 2003). As explained earlier, CXCR7 was shown to act as a chemokine sink, removing excess CXCL12 from the extracellular space and, in the process, preserving the developing interneurons' responsiveness to CXCL12-mediated chemotaxis (Sanchez-Alcaniz et al., 2011). Although this function does not involve the immune system, the signaling outcome of chemotaxis is similar to outcomes observed with inflammatory chemokines, suggesting that this chemokine/receptor pair is more ancient and more important because of its presence in organisms that do not have a functional immune system (Huising, Stet, Kruiswijk, Savelkoul, & Lidy Verburg-van Kemenade, 2003). Additionally, CXCR4 knockout animals do not survive past birth, further indicating the importance of this receptor during development (Ma et al., 1998). In the mature CNS, CXCL12 may also have other functions that are more diverse than chemotaxis. In periods of neuronal stress or excitotoxicity, CXCL12 can protect neurons by several different mechanisms, as discussed later.
The importance of the CXCL12/CXCR4 signaling axis becomes much more apparent in pathological states where the signaling is dysregulated, including HIV infection. HIV can infect peripheral immune cells and use them as a liaison to enter the CNS (Gonzalez-Scarano & Martin-Garcia, 2005). Upon entering, the virus can establish a CNS reservoir and by extension cause activation of CNS immune cells and an inflammatory response (Kraft-Terry, Buch, Fox, & Gendelman, 2009). Both cells that are infected and uninfected can contribute to the inflammatory/excitotoxic state, and the HIV proteins themselves can have detrimental effects on many different CNS cell types (recently reviewed by Gonzalez-Scarano & Martin-Garcia, 2005; Lindl, Marks, Kolson, & Jordan-Sciutto, 2010). The HIV-envelope protein from X4 viruses uses CXCR4 for entry into cells, but it can also cause cell damage via CXCR4-dependent signaling events (Pandey & Bolsover, 2000). Depending on the cell type expressing CXCR4, this binding event can result in different outcomes. For example, gp120-induced activation of CXCR4 in glia can cause secretion of several inflammatory mediators, including tumor necrosis factor-α and interleukin-1β (Bezzi et al., 2001). These mediators can activate nearby uninfected immune cells and precipitate inflammatory responses in the CNS (Kraft-Terry et al., 2009). Astrocytes are the main support cells in the CNS, and their typical functions can be altered in HIV infection, even though these cells are not the primary target of HIV in the CNS. Inflammatory mediators can alter their ability to prevent excessive stimulation of glutamate receptors in the tripartite synapse, resulting in the formation of an excitotoxic environment (Okamoto, Wang, & Baba, 2005). Additionally, gp120 can directly bind CXCR4 on neurons, which contributes to neuronal demise and simplification (Bardi, Sengupta, Khan, Patel, & Meucci, 2006; Ellis, Langford, & Masliah, 2007; Hesselgesser et al., 1998; Meucci et al., 1998). In summary, while activation of CXCR4 by its natural chemokine ligand, CXCL12, is generally neuroprotective, abnormal stimulation of this receptor during HIV infection can have opposite effects in the CNS.
Patients infected by HIV who also abuse opioid drugs often show enhanced disease progression at the periphery and in the CNS, so an examination of the interaction between the opioid and chemokine system in the brain can provide valuable insights for understanding this comorbid pathology. Interestingly, DAMGO, a potent and selective μ-opioid receptor agonist, was shown to inhibit neuroprotection afforded by CXCL12 treatment in NMDA-treated neuronal cultures (Patel et al., 2006). This effect was associated with DAMGO's ability to prevent ERK and Akt phosphorylation via CXCL12 signaling. CXCL12 is also able to directly modulate NMDA receptor subunit composition on cortical neurons, by preventing the production (and likely insertion) of the NR2B receptor subunit at extrasynaptic sites (Khan et al., 2008; Nicolai, Burbassi, Rubin, & Meucci, 2010). This subunit is associated with enhanced Ca2 + currents and overstimulation of the neuron, resulting in a disruption of ionic homeostasis, and potential activation of caspases (Leveille et al., 2008).
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Influence of Bone Marrow Microenvironment on Leukemic Stem Cells
Puneet Agarwal, Ravi Bhatia, in Advances in Cancer Research, 2015
4.1.4 CAR Cells
CXCR4 is commonly expressed by leukemic cells of both myeloid and lymphoid lineage and SDF-1 (usually called CXCL12) is its ligand which is released by the BMM. This CXCL12/CXCR4 interaction has been shown to be critical in the retention of LSC in the BMM (Kremer et al., 2014; Tavor et al., 2004). CXCR4 antagonist such as Plerixafor (AMD3100) has shown promising results in disruption of LSC from the BM lodgment, hence making them more accessible to conventional therapeutics (Becker, 2012). Studies of mice with the GFP gene knocked into the CXCL12 locus identified reticular cells with high expression of CXCL12 termed CAR cells, scattered throughout the bone marrow with long processes creating a network. CAR cells have the ability to differentiate into adipocytes as well as osteoblasts and to act as niches for HSC. Ablation studies indicate that they have the ability to support proliferation of B cells, erythroid cells and HSC, and to maintain HSC in the undifferentiated state. CAR cells localize perivascularly next to the BM sinusoidal endothelium in human BM; CD146 expressing subendothelial cells appear to be the counterpart of CAR cells.
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Pharmacology of G Protein Coupled Receptors
Miles Congreve, ... Fiona H. Marshall, in Advances in Pharmacology, 2011
D Chemokine Receptor CXCR4
CXCR4 is one of 19 human chemokine receptors and is activated by the large peptide ligand SDF-1 (CXCL12). The primary role of chemokines and their receptors is in leukocyte trafficking to sites of inflammation as well as cell homing and patterning during development. CXCR4 is of particular interest as it has been implicated in a number of cancers (Rubin, 2009) and is a target for HIV-1 cell entry (Feng et al., 1996). Plerixafor is an antagonist of CXCR4 that has been approved for use in stem cell mobilization (Cashen, 2009). However, this compound was identified from an antiviral assay and was not optimized for activity at CXCR4. It has poor drug-like properties, such as a lack of oral bioavailability and toxicity associated with its potential for metal ion chelation (Este et al., 1999). The development of improved CXCR4 antagonists would be potentially very useful for a wide range of diseases.
The structure of CXCR4 has recently been obtained in complex with both cyclic peptide and small molecule antagonists (Wu et al., 2010a). This is the first peptide GPCR structure to be solved and so represents a major breakthrough in understanding the diversity of GPCR structures and for modeling chemokine and other peptide receptors for drug discovery.
The CXCR4 receptor structure was obtained using a construct that included both the T4L fusion as well as a number of thermostabilizing mutations (L125W (3.41) and T240P (6.36)). The antagonists used for crystallization were IT1t (an isothiourea) and CVX15, a 16-residue cyclic peptide antagonist. Five different structures have been published which differ in the truncation of the C-terminus, the presence or absence of the T240P mutation and the ligand (IT1t or CVX15). All structures include the L125W mutation and the T4L fusion. The N-terminal 26 residues are not visible in the structure and are presumed to be disordered.
The structures have a number of interesting features not previously seen in other GPCR structures. Although the overall fold of the helices is the same, TM1 in particular is shifted toward the core helical bundle compared to the aminergic receptors. This may be a feature of peptide receptors in which ligands bind primarily to the N-terminus of the receptor but must still engage the transmembrane domain to trigger the conformational changes associated with receptor activation. Other differences include a rotation in the extracellular end of TM2 resulting from a tighter turn around the conserved proline (2.58), significant differences in the positions of the ends of TM4, and a shift in the extracellular end of TM6 compared to the β-adrenergic and adenosine A2A receptors.
The intracellular C-terminus of CXCR4 differs significantly to other GPCR structures. TM7 is one turn shorter ending in the NPxxY motif, and there is no final α-helix in the C-terminal tail; this is usually called helix 8. CXCR4 does not contain the full α-helical motif usually present in this region and does not have an obvious palmitoylation site which would tether the C-terminus to the plasma membrane. Therefore, the possibility exists that CXCR4 and other chemokine receptors do not have the usual helix 8. However, this may be an artifact of the crystallization conditions or the constructs used in these studies. This is an important region with regard to drug discovery, as a number of allosteric modulators have been identified to related chemokine receptors which have been shown to bind to an intracellular binding sites in the region of helix 8. For example, SB265610 (1-(2-bromo-phenyl)-3-(7-cyano-3H-benzotriazol-4-yl)-urea) behaves as an allosteric inverse agonist of CXCR2. This compound is sensitive to mutations K320A, Y314A in the C-terminal tail and D84N(2.40) in TM2 (Salchow et al., 2010). A similar binding site has been reported for other CXCR2 antagonists (Nicholls et al., 2008) and is present in CCR4 and CCR5 receptors (Andrews et al., 2008).
CXCR4 is the first peptide activated GPCR to be crystallized, and the costructures with both peptide and nonpeptide antagonists demonstrate how such ligands can block the activity of the much larger peptide agonist CXCL12. A two-site model of activation or a message-address concept has been suggested to explain the binding of large peptide or hormone ligands. "Site one" represents the address or ligand recognition site; in the case of opioids and neurokinins, this site determines the specificity of ligand/receptor interactions (Portoghese, 1989; Werge, 1994). In peptide receptors, this usually consists of extracellular regions of the protein including the N-terminus and/or extracellular loops (ECL). NMR structures of the N-terminus of CXCR4 in complex with SDF1 have been determined (Veldkamp et al., 2008). These regions involve high affinity multivalent binding interactions between the receptor and hormone which would likely be difficult to disrupt with small molecule antagonists. In contrast, "site two" or the message region is the key interaction which triggers receptor activation and signaling (Clark-Lewis et al., 1995). Residues in the N-terminus of the peptide make contact with the core of the 7TM region—a drug-like site analogous to the orthosteric binding site of aminergic receptors.
In CXCR4 residues Asp187 (in ECL2), Glu288 (7.39), and Asp97 (2.63) which are important for the binding of the N-terminus of CXCL12 also interact with the small molecule antagonist ITI1t as well as the peptide CVX15. Indeed, it is possible for functional antagonists to block receptor signaling by blocking binding to these regions without displacing binding interactions to the N-terminal domain (Kofuku et al., 2009). This core 7TM region likely represents the most druggable binding site for Family A GPCRs, however, this may lead to a lack of drug selectivity across what may at first appear relatively unrelated receptors. For example, antagonists of the CCR3 receptor are structurally related to dopamine receptor antagonists (Faure et al., 2010). The conservation of residues within the core GPCR binding site can be used to design ligands for peptide GPCRs. For example, nonpeptide antagonists for the somatostatin SSTR5 receptor were derived from the histamine H1 antagonist astemizole using a chemogenomics approach which compared amino acids in the consensus drug-binding pocket (Martin et al., 2007).
Figure 2 shows an overlay of representative antagonist GPCR protein–ligand structures for β1 and β2-adrenergic, A2A, CXCR4, and D3 receptors. The figure illustrates that, despite the fact that there are significant differences in the key interactions from system to system, each of the receptors binds their ligands in the TM domain in a colocated, overlapping, contiguous set of binding sites. The TM domain site overall has a tractable topology, very encouraging as to the potential for structure-based drug design (SBDD), described later in this chapter.

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Figure 2. Comparisons of antagonist binding sites in GPCR crystal structures. An overlay of GPCR structures with antagonist ligands bound showing common position of ligand binding pocket. Structures are the β1 adrenerigic receptor (2VT4, blue), β2 adrenerigic receptor (2RH1, orange), adenosine A2A receptor (3EML, red), CXCR4 (3ODU, brown), and dopamine D3 (3PBL, pink).
I keep the nood IDD on my hand it isn't danger